There are many presently known panoramic and fish-eye viewing optical arrangements which use lenses, or optical blocks, of various shapes. Examples of panoramic arrangements are described in U.S. Pat. No. 2,638,033 to Buchele et al; U.S. Pat. No. 3,552,820 to Brachvogel; U.S. Pat. No. 4,395,093 to Rosendahl et al; U.S. Pat. No. 4,484,801 to Cox: U.S. Pat. No. 4,566,763 to Greguss, while examples of fish-eye systems are described in U.S. Pat. Nos. 4,256,373 and 4,412,726 both to Horimoto.
Further, an arrangement consisting of a concave reflecting surface and a convex reflecting surface facing each other is known as a Schwarzschild system.
Each of these has its own advantages and disadvantages. While both the more conventional fish-eye lens and the panoramic block arrangement might work satisfactorily for certain applications, their use becomes problematic when attempting to design an optical system which has both a large image format and a large backworking distance. For example when a panoramic arrangement is appropriately scaled up for handling the larger image format, the size of the block becomes not only bulky but depending on the material could be extremely expensive to produce, thus making the panoramic block a less than attractive solution. As for the fish-eye lens, since the focal length associated with it tends to be small compared to its size in order to obtain the proper object to image mapping, obtaining sufficient backworking distance is usually at the expense of greater complexity and performance. While a ratio of backworking distance to focal length in the region of 2:1 is not uncommon, increasing this to 3:1 imposes a severe burden on the design.